Check wich versions of Node are compatible with Duniter

Here: https://forum.duniter.org/t/nouveau-tutoriel-de-developpement/3570/11, we have a user that can't use Duniter with NodeJS 9.

I think Duniter should be compliant with Node 6, 7 and 8, but let's check this.

Also once it's done, change the package.json to tell this. I finally notice that current engine constraint in package.json is too hard:

"engines": {
  "node": ">=6.11.1",
  "npm": ">=3.10"
}

We should just say ">=6" for example.